1. Field of the Invention
The present invention relates to a heart rate detector system, and more particularly, to an improved heart rate detector capable of use with an implantable defibrillator for defibrillating the heart of a patient when the patient experiences a life-threatening arrhythmia.
2. Description of the Prior Art
In recent years, substantial progress has been made in the development of defibrillation techniques for providing an effective medical response to various heart disorders or arrhythmias. Earlier efforts resulted in the development of an electronic standby defibrillator which, in response to detection of abnormal cardiac rhythm, discharged sufficient energy, via electrodes connected to the heart, to depolarize the heart and restore it to normal cardiac rhythm. Examples of such electronic standby defibrillators are disclosed in commonly assigned U.S. Pat. Nos. 3,614,954 (subsequently, Re. 27,652) and 3,614,955 (subsequently, Re. 27,757).
Past efforts in the field have also resulted in the development of implantable electrodes for use in accomplishing ventricular defibrillation (as well as other remedial techniques). In accordance with such techniques, as disclosed (for example) in U.S. Pat. No. 4,030,509 of Heilman et al., an apex electrode is applied to the external intrapericardial or extrapericardial surface of the heart, and acts against a base electrode which can be either similarly conformal or in the form of an intravascular catheter. Such electrode arrangements of the prior art, as disclosed in the aforementioned patent of Heilman et al., can employ independent pacing tips associated with either a base electrode or an apex electrode, or both.
Recent efforts also have resulted in the development of techniques for monitoring heart activity (for the purpose of determining when defibrillation or cardioversion is necessary), which techniques employ a probability density function for determining when ventricular fibrillation is present. Such a technique, employing the probability density function, is disclosed in U.S. Pat. Nos. 4,184,493 and 4,202,340, both of Langer et al.
In accordance with this latter technique of the prior art, when the probability density function is satisfied, fibrillation of the heart is indicated. However, recent experience has shown that, with certain unusual ECG patterns, the prior art probability density function detector, if not optimally adjusted, can be "triggered" not only by actual ventricular fibrillation, but also by some forms of high rate ventricular tachycardia, and low rate ventricular tachycardia as well, particularly in the presence of ventricular conduction abnormalities. The possibility of such triggering in the presence of some high rate tachycardia is acceptable because high rate tachycardia often can be fatal if present at such a rate that sufficient blood pumping no longer is accomplished. However, triggering in the presence of non-life threatening, low rate tachycardia could be considered a problem. Therefore, it has been determined that there is a need for a system and method for distinguishing between ventricular fibrillation and high rate tachycardia, on the one hand, and low rate tachycardia, on the other hand.
One response to the need discussed above is described in commonly assigned co-pending patent application Ser. No. 175,670 filed on Aug. 5, 1980, entitled "Arrhythmia Detection System And Method" (Langer et al.). There, a probability density function circuit, responsive to differentiated ECG signals from the heart electrodes, is used in conjunction with a heart rate circuit whereby the probability density function (PDF) circuit activates a defibrillator pulse generator only when the PDF circuit is enabled by the heart rate circuit. Such enabling occurs when the heart rate exceeds a predetermined value reflecting what is considered to be a dangerous high rate tachycardia.
The success of the latter system depends, in large part, on the reliability and accuracy of the heart rate detector circuit. Heart rate detectors, per se, are known in the art. Such heart rate detectors are typically designed to be responsive to incoming ECG waveforms of a predetermined type. For example, it is known to detect a heart rate by the use of a zero-crossing detector. In such detectors, the zero-crossing points of the ECG waveform reflect a periodic event in the cardiac cycle. When, however, the ECG waveform is characterized by rather steep slopes, for example when the rate of change of the R-wave voltage is steep, or spiky, then the use of such a system to detect the zero-crossings loses accuracy. The steep slope R-wave complex, with its accompanying Q and S segments, results in multiple counts per cardiac cycle, giving an artificially high rate reading, which could, in certain cases, be significant.
It is also known in the art to provide a heart rate detector responsive to those ECG signals having steep or "spiky" slopes. Some such detectors respond to the ECG signal and provide an output responsive to such steep slope signals. Such output may be provided by a slew rate detector which compares the slope, or slew rate, with a slew rate threshold and provides an output signal reflecting the number of high slew rate signals detected. A problem inherent in such a system is the detection of a heart rate when the ECG signal is more sinusoidal than spiky. In such cases, the slew rate, or rate of change of the ECG voltage versus time, characteristic is small. Thus, the detector may not pick up such signals, thus, resulting in detection of an inaccurate low heart rate.
In very ill patients, it is not unusual to find that the ECG waveforms change from time-to-time. The ECG could present itself as a spiky waveform for a time, and then become more sinusoidal, or vice versa. Rate detectors, of the types described above, would not be versatile enough to effectively respond to both types of ECG waveforms.
It is thus seen that the prior art heart rate detectors fail to provide the required flexibility in monitoring ECG signals characterized by both spiky ECG waveforms and the more sinusoidal ECG waveforms. Prior art detectors can be designed to work very efficiently with one or the other of such waveforms, but not both. Therefore, it has been determined that a need exists to provide a flexible, accurate and reliable heart beat rate detector that is operable over a broad range of detected ECG waveforms.